Optical detection methods are frequently employed in analytical instruments for sensitive detection of chemical compounds, such as biomolecules. Optical detection is particularly well suited for analytical techniques where the absorption or the concentration of a substance are measured or determined, such as liquid chromatography or filtering systems, which are important tools in biotechnological, biomedical and biochemical research as well as in the pharmaceutical, cosmetics, energy, food and environmental industries.
Typically, the optical systems associated with liquid chromatography use a series of lenses and mirrors to collect and propagate light from a light source to a flow cell, through which a fluid containing a sample is allowed to flow. Light not absorbed after passing through this cell strikes a light detector containing one or more photosensitive elements and is subsequently analyzed to provide information regarding the sample.
Often, and in order to achieve satisfactory results of the analyses, optical fibers are used for propagating light within the system. This has the advantages of minimizing the use of optical elements such as collimating lenses and reflectors, as well as providing a flexible transfer of lights and a minimal loss of intensity, even over extended distances.
The light source can be a xenon lamp, for instance, that emits flashes of light at certain intervals. Due to factors such as fluctuation of intensity, baseline drift and temperature drift, it is often necessary to arrange the optical fiber in a series of bends and loops to allow for a coupling of propagation modes that serves to provide a smooth output of light, regardless of fluctuations of the input from the light source. Such an arrangement of the optical fiber can be denoted as a mode coupling fiber arrangement. Especially for applications where a beam splitter is used to split the light into two portions, in order to provide a stable reference portion that is not transmitted to the flow cell, it is important that the output from the optical fiber is as uniform as possible in order to achieve satisfactory results.
Methods for mode coupling within the field of liquid chromatography are known in the art, for instance through loops, microbends or notches. Examples of such solutions can be found in U.S. Pat. No. 6,963,062 (Eksigent Tech, LLC), U.S. Pat. No. 7,945,130 (General Photonics Corp.) or U.S. Pat. No. 4,676,594 (American Telephone & Telegraph).
The mode coupling or mode mixing arrangement commonly used has the disadvantages of being bulky and requiring a relatively long optical fiber to allow for a sufficient coupling of optical modes.
Through the loops and bends, transmission losses also occur, especially in the deep UV region below 250 nm. If the bend radius is kept large, the losses are smaller but the mode coupling ability is decreased, requiring an even longer fiber in order to achieve an output of sufficient quality.
The problems associated with liquid chromatography as described above are also applicable to other systems for measuring the absorption or determining the concentration of a substance, where a stable reference value is required. Such systems comprise filtering systems, cell harvesting systems, clarification systems, formulation systems and similar systems for production of biopharmaceuticals, among others.
There is therefore a need for an optical fiber arrangement with improved mode coupling suitable for such systems without the disadvantages described above.